Image converter tube



March 25, 1941. F. COETERIER IMAGE CONVERTER TUBE Filed Oct. 5, 1959 INVENIOR. FREDER/K COETER/ER www ATTORNEY.

Patented Mar. 25, 1941 UNITED STATES PATENT OFFICE IMAGE CONVERTER TUBE Netherlands Application October 3,

1939, Serial No. 297,652

In the Netherlands October 13, 1938 5 Claims.

My invention relates to image converter tubes and particularly to tubes by which radiant energy images may be intensified or transformed to radiant energy images having different characteristics.

It is possible to transform light of one wavelength into light of another wavelength, and to this end a picture may be formed by projecting primary rays of light or radiant energy on a photo-electric surface or picture cathode in an electric discharge or image converter tube to liberate electrons which are focused on a fluorescent viewing screen.

If the picture cathode and the fluorescent screen are arranged exactly opposite one another, particular precautions must be taken to obviate the disadvantages that the light which falls on the picture cathode is partly intercepted by the fluorescent screen thereby increasing the background illumination and that the view of the fluorescent screen is not obstructed by the picture cathode. However these particular precautions cause further difficulties and therefore it has been proposed to arrange the picture cathode and the fluorescent screen at an angle with respect to one another. By means of a magnetic field whose lines of force substantially extend in the direction of the electron paths, the electrons may be caused to travel along curved paths and to reach the fluorescent screen, where, owing to the focus ing effect of the magnetic field, a well-defined picture may be produced. It is, however, impossible to attain in this way a great reduction in the size or area of the picture. This is a drawback of magnetic focusing because it is impossible to focus electrons from an electron source on an area smaller than thesource. However, in the transformation of the picture it is very desirable to reproduce the picture in a smaller size so that the intrinsic brilliance of the picture may be increased.

It is an object of my invention to provide an image converter tube wherein the area of the converted image may be varied at will with respect to the area of the original optical image while at the same time background illumination is minimized. It is a further object of my invention to provide a structure to increse the intrinsic light intensity of an image representative of an optical image in an image converter tube.

In accordance with my invention the electrons issuing from each point of the irradiated surface of the picture cathode of an image converter tube are focused to form a picture of reduced area by establishing the electron discharge inside a spherical or substantially spherical electrode. The spherical electrode is preferably transparent or provided with windows for the passage of light. In operation the spherical electrode of my tube is given a potential which is negative with respect to the picture cathode so that the electrons which are emitted by the picture cathode owing to the irradiation of the latter are accelerated in the electric field between the cathode and the fluorescent screen and are concentrated on and directed to this screen where they form a well-defined reduced reproduction of the picture projected on the cathode. Furthermore the picture may be sharply focused by varying the potential of the spherical electrode.

In accordance with one modification of my invention I provide an image converter tube in which still further reduction in picture area may be obtained by arranging between the picture cathode and the fluorescent screen a specially positioned annular electrode connected to a variable potential source. This electrode will hereinafter be referred to as the first accelerator. By an adequate choice of the potential of this first accelerator it is possible to ensure that the field strength in the neighborhood of the fluorescent screen increases considerably with the result that a strong refracting eifect is exerted on the electrons from the cathode. By modifying the potential of the first accelerator it is possible to control still further the reduction in size of the converted image.

The invention will be more clearly understood by referring to the accompanying drawing, which represents, by way of example, a discharge tube i and circuit connections made in accordance with my invention.

Referring to the drawing the tube comprises an evacuated envelope or bulb I, one portion of which is preferably spherical with an elongated neck section closed at the end opposite the spherical portion by a conventional press. The bulb I encloses a photo cathode or picture cathode 2, preferably concave on the side facing the center of the spherical portion of the bulb and a fluorescent screen foundation electrode 3 exposed to the cathode 2 but preferably in a plane normal to the projected surface of the cathode. The cathode 2 may be made of nickel or silver while the foundation electrode 3 may be of molybdenum or other refractory metal. On the side of the electrode 3 exposed to the cathode I provide a fluorescent screen 4 of fluorescent material such as zinc silicate or zinc sulphide which fluoresces when bombarded by electrons.

The cathode 2 if not made of silver is coated with a layer of silver which is superficially oxidized and subjected to caesium as well known in the art and serves as an electron emitting cathode when connected to the negative terminal of a potential source 5 whose positive terminal is connected to the foimdation electrode 3, which acts as an anode. The spherical section of the bulb l is coated on the inside with a layer 6 of conductive material, such as silver, which is in electrical contact with a supply lead 1 and constitutes the spherical electrode. This silver layer may be provided simultaneously with the silver layer on the cathode 2, such as by vaporizing silver within the envelope from a source, not shown. The lead 1 is connected to a potentiometer 8 which is connected in parallel with a potential source 9, and by means of this potentiometer the layer 6 may be given a potential which is negative with respect to the cathode 2. Since the layer 6 consists of conductive material such as silver and is energized from a single source such as the lead I, the layer is at a single potential over its entire surface and may be referred to as a unipotential layer or electrode.

Two openings or windows It and H are left in the coating 6, and an optical image of the object to be observed is formed on the cathode 2 with the aid of a suitable optical system using either visible or invisible reflected light from the object. While I have shown the layer 6 as being discontinuous at the windows in and H the layer 6 ma be of the semi-transparent type, this being preferable since theelectrostatic field is more uniform if the layer 6 is continuous over the window portions of the envelope. Since the cathode 2 is made photo-electrically emissive, it emits electron streams constituting an electron image having a current density varying with the areas of light and shade on the cathode.

Owing to the combination of cathode, fluorescent screen, anode and the negative spherical electrode the electron streams constituting an electron image of the original optical image are concentrated, the electron image being directed on the surface of the fluorescent screen to form a picture on the screen. This picture is a reversed image, that is to say, the electron streams issuing from the various points of the cathode intersect each other, and this picture may be observed through the window if desired, with the aid of a magnifying lens system.

In contradistinction to the known apparatus the picture on the fluorescent screen 4 is greatly reduced, the degree of reduction depending on the size of the bulb and on the voltage applied. Thus, for example, with a bulb having a diameter of 10 cms. and with a voltage of 5000 volts between the cathode and the anode 3 a reduction of 3:1 has been obtained. The potential of the spherical electrode or layer 6 with which a well-defined picture is obtained is in this case about 100 volts negative with respect to the cathode. The optimum value of the potential can easily be determined experimentally.

In order to increase the degree of reduction of the picture and to be able to control said degree I interpose, between the cathode 2 and the anode 3, a first accelerator or annular electrode l2 which surrounds the streams of electrons from the cathode 2. This electrode, which acts as a first accelerator, has a separate current supply conductor I3 which, as the supply conductor M of the anode, is led through a pinch 15 of the stem N5 of the tube. By means of the conductor [3 the annular electrode [2 is connected to a potentiometer I! which is connected in parallel with the source of current 5. It is thus possible to apply to the electrode [2 a potential lying between that of the cathode 2 and that of the anode 3. Variation of this potential permits control of the degree of reduction in the picture area on the fluorescent screen 4. The reduction is greater according as the potential of the first accelerator or annular electrode l2 comes nearer to that of the cathode.

In the example above referred to it is possible to obtain with a first accelerator potential of 20 volts a reduction of 6:1 and consequently an increase of brightness of 36 with respect to the magnetically focused picture.

In the drawing l8 denotes a capsule which is also arranged on the pinch I5 and serves to activate the cathode 2 in the known manner with a suitable photo electrically sensitive metal such as caesium. Upon evaporation of the caesium from the capsule into the high vacuum the caesium atoms liberated cannot strike the fluorescent surface of the anode. They exclusively attain the cathode and wall of the bulb. The cathode, which is coated with a layer of silver oxide, thus becomes photo-electric. That proportion of the vaporized caesium which does not strike the cathode is retained by the coating 6.

The arrangement of the electrode 3 or fluores- 0 cent screen t with respect to the cathode need not be such, as in the example shown, that the axes of these two electrodes are exactly perpendicular to one another. They may also make smaller angles with one another without this having any influence on the desired results.

The annular electrode 12 should preferably be given a slightly oblique position so that the plane through the electrode is at an oblique angle to the plane of the anode 3 or fluorescent screen 4, as is shown in the drawing, in order to ensure that the electron picture is completely similar to the luminous picture on the cathode. The first accelerator must be given a more oblique position with respect to the anode according as it is more remote from the anode 3. In order obtain an efficacious reduction, it must not be too remote from the anode and at any rate it must be closer to the latter than to the cathode.

While I do not wish to be limited to any particular theory to explain the improved operation of my device, it will be found, as a result of plotting the electrostatic field distribution adjacent the photo-cathode, that the equipotential lines or surfaces immediately adjacent the concave cathode are likewise concave with respect to the center of the envelope. However, the combined electrostatic field created by the spherical unipotential layer 6 and the anode 3 distort and flatten the field adjacent the photo-cathode so that the equipotential surfaces in front of the cathode have a larger radius of curvature and are in part reversely curved. In fact, the electrostatic surfaces have a curvature which varies with the distance from the anode 3, and since these equipotential surfaces vary in curvature, the streams of electrons emitted by the cathode are bent in their passage from the cathode to the screen 4. The paths I9 and 20 Show the electron paths from two representative points on the cathode. A variation in the potential difference existing between the spherical unipotential layer 6 and the cathode 2 causes the length of the electron paths between the cathode and screen to vary and consequently the focus of the electron image on the screen 4 may be adjusted by varying this potential difference.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only one specific application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing from the scope of my invention as set forth in the appended claims.

What I claim as new is:

1. An image converter tube comprising an evacuated envelope, a photo cathode adapted to receive an optical image, a luminescent anode exposed to said cathode, and a unipotential spherical electron focusing electrode enclosing said cathode and said anode and adapted when at a negative potential with respect to said cathode to focus the electron discharge from said cathode upon said anode and to direct on the surface of said anode an electron image corresponding to the optical image on said cathode.

2. An image converter tube comprising an evacuated envelope enclosing a cathode adapted to have formed thereon an optical image of an object outside the envelope, a planar anode exposed to said cathode and substantially normal to the intial direction of electrons emitted by said cathode, and a spherical unipotential electrode enclosing said cathode and anode and having portions which are pervious to the passage of light to the cathode and anode to focus the electron discharge from said cathode upon said anode.

3. An image converter tube comprising an evacuated envelope having a spherical portion, a cathode having a concave surface facing the center of said spherical portion within said envelope, a luminescent anode to become luminescent when bombarded with electrons facing the center of said spherical section and a unipotential concluctive coating on the inner surface of said spherical portion of said-envelope to direct electrons emitted from areas on said cathode to corresponding areas on said anode.

4. An image converter tube comprising a spherical envelope section, a unipotential electrode coating of electrically conductive material on the interior surface of said spherical section, a concave electron emitting cathode having a radius of curvature substantially the same as said spherical section closely adjacent a portion of the said electrode coating, the concave surface of said cathode facing the center of said spherical section, a fluorescent anode located substantially at right angles with respect to said cathode and opposite the center of said spherical section to receive electrons from said cathode, and an annular anode surrounding a small portion of the path of the electrons from said cathode to said anode positioned in a plane which intersects the plane of said anode obliquely to focus said electrons upon said fluorescent anode.

5. An image converter tube comprising an envelope having a spherical portion, a unipotential electrically conducting coating on the interior surface of said spherical portion, a cathode in the spherical portion of said envelope and having a concave surface facing the center of said spherical portion, a luminescent target having an extended surface in said envelope facing the center of said spherical portion and removed from the direct path between said cathode and the unipotential electrically conducting coating through the center of said spherical portion and an annular anode lying in a plane making an oblique angle with the plane of the extended surface of said target to focus electrons from said cathode upon said target.

FREDERIK COETERIER. 

